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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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YAKUTAT BAY GLACIERS                                      29
surface, with few crevasses. Therefore, once the crevassed ice that is often present below the snow line is passed, these through glaciers form excellent highways. Those to the east of Yakutat Bay were followed by many prospectors, in 1898-1899, as the most feasible route to the Alsek valley, and their sledges and other relics of this over-ice travel may still be seen on and near the terminus of Nunatak Glacier, Fourth Glacier, and at the head of Russell Fiord.
Feeding the large through glaciers, both in the ice-submerged divide areas and in their lower courses, are many valley glacier tributaries which descend from, the mountains that rise above the ice flood. None of them are as yet named, and none of them have so far been studied, excepting only those on the slopes of Mt. St. Elias. They appear to offer no unusual features, and nothing is known that distinguishes them from ordinary Alpine glaciers above snow line. Some are small and steep, others long and with moderate slope, and many increase in steepness near their lower ends, frequently forming ice cascades as they descend from their valleys to join the main ice mass. The latter condition, characteristic of the majority of tributary glaciers in this region, is due to former glacial erosion which lowered the main valley and left the tributary valleys hanging; for great though the expanse of snow and ice in this region is at present, it is far less than formerly existed, even in the supply areas. Indeed, in some cases, it is evident that present-day through glaciers, now flowing in opposite directions, were formerly continuous glaciers, flowing in one direction across the divide. It is, in part at least, as a result of this former condition that the divide areas have been so lowered by glacial erosion that they are now flat enough to be drowned by ice and snow and serve as the distributing area of a system of radiating glaciers. At present there is a network of glaciers flowing in various directions in the depressions between the higher peaks; formerly tin's network was even more extensive but the direction of flow was often changed and even reversed.
The ice supply of these through glaciers comes from three principal sources, (1) directly from snowfall, since the snow line lies between 2000 and 3000 feet in the area of the through glaciers, (2) from avalanches, (S) from tributary glaciers. From the last two sources are supplied rock fragments as well as snow, and these are incorporated throughout the ice, though naturally in greatest abundance near the glacier margins and along bands medially placed by the junction of glaciers. A third source of rock fragments is through erosion by the ice itself, and this debris is mainly concentrated in the bottom layers. From these several sources the Alaskan glaciers receive a large supply of debris; but this is not apparent above the snow line where the white, snow-covered surface (PL IV) shows the presence of rock fragments only occasionally where a freshly fallen avalanche has not yet been buried beneath snow. Below the snow line, however, the presence of abundant rock fragments' becomes apparent, where, by ablation, they become concentrated on the ice surface, often completely covering the glacier in its lower portion.
The abundant supply of rock fragments from avalanches is dependent upon at least five favorable conditions, as follows: (1) weathering, which is very active in these high mountains, whose steeper slopes are exposed to sharp changes in temperature; () the abundant snowfall which, forming deep accumulations on the mountain slopes, and attaining a condition of instability, is all the time sliding down the slopes and by its erosion tearing off portions of the mountain itself; (8) the condition of the rock, which in